1. Field of the Invention
This invention relates to improvements in stacked multicell batteries, and more particularly to preventing unwanted ionically conductive paths from forming between adjacent cells and the battery case.
2. Description of the Prior Art
When constructing practical electrochemical cells in batteries there are two basic ways in which electrodes can be connected inside the cell or battery module case. These are series and parallel connections. In a bipolar battery design, the electrodes are hooked together in series, thus the voltage of the stack is n times that of a single cell, where n is equal to the number of cells in the stack. Each cell comprises a positive and a negative electrode physically separated from one another by an ionic conductor.
The positive electrode, negative electrode and separator of each cell contains an electrolyte which is liquid at the operating temperature. The bipolar wall separates adjacent cells and is designed to allow an electronic path between adjacent cells while not allowing an ionic path. Positive and negative current collecting plates are typically provided at opposite ends of the cell stack.
It is well known in the battery industry that when porous, high surface area electrodes in an electrochemical cell or battery are charged or discharged they undergo volume changes due to the formation of different chemical species as a result of the oxidation and reduction reactions. If maximum performance is to be realized from an operating battery over an extended period of time, good electrical contact must be maintained at all times between the electrodes and current collection system, in spite of the electrode volume changes. The good electrical contact is achieved in principle by applying sufficient uniform pressure to the electrode/current collector assembly. In ambient temperature batteries, the pressure is usually achieved by stuffing the electrode/current collector assembly into a rigid cell case before adding the liquid electrolyte. The addition (or stuffing) of the electrolyte typically causes some swelling of the electrodes and hence a uniform pressure is established on the electrode/current collector assembly.
Unfortunately this stuffing technique cannot be readily applied to high temperature molten salt electrolyte batteries in which the electrolyte is incorporated into the powder electrodes and separator during their manufacture. Consequently, a more elaborate technique is necessary to apply this uniform pressure and maintain it at temperatures in the range of 400.degree. to 500.degree. C. (depending on the melting point of the alkali-halide electrolyte chosen).
An additional problem that develops with lithium alloy/metal sulfide high temperature molten salt electrolyte batteries is that due to the nature of the very reactive and highly hygroscopic lithium compounds they contain, the cells/batteries must be well sealed from the outside air (i.e., oxygen and moisture) to prevent any undesirable reactions and, hence, loss in cell/battery performance. The difficult problem in affecting a sealed cell/battery is that the seal must also function to electrically isolate one of the cell/battery terminals from the other terminal of opposite polarity.